The objective of this is to elucidate the basic mechanisms involved in the regulation of erythropoiesis and apply this knowledge to human erythropoietic diseases. Partially purified preparations of erythropoietin markedly stimulate an increase in the concentration of cyclic GMP in fetal liver erythroid cells. We have now shown that endotoxin is the substance in the erythropoietin prepations that increases cyclic GMP. Studies are now being conducted to determine the precise role of elevated cyclic GMP in the action of endotoxin on cells. The polycythemia produced by the Friend virus also is studied. We have found that Friend virus plasma can be added to normal mouse marrow cells in vitro in the plasma clot system, and will produce a large number of erythroid bursts at 4 days of culture. This effect does not appear to be due to erythropoietin in the plasma since the activity sediments with virus is sucrose gradients and is heat labile, whereas the hormone is not affected by heat treatment. We now intend to establish the optimum conditions for this in vitro transformation. In addition, the methods developed in these investigations will be applied to the study of polycythemia vera.